Dielectric sensors are used in many applications. For example, without limitation, they can be used to measure liquid density of a medium or as liquid level sensors. The density of a substance or substances with known dielectric constant can be inferred from the measured dielectric constant of a medium. Water has one of the highest dielectric constants and so dielectric sensors are particularly well suited at measuring the density and level of water. For purposes of this invention, a “liquid level sensor” is a sensor which will establish the relative location to which a given liquid fills a container (e.g. the top of the fluid in a tank). Such containers may be manmade or natural and may include channels in which liquids flow. The liquids contained therein may be static or moving. Whereas a “moisture content sensor” will determine the amount of moisture (which may be from any type of liquid) in a given sample of a medium.
The dielectric constant of a medium can be measured in various ways. The dielectric constant of a material can affect capacitance of capacitor plates, and the characteristic impedance of transmission lines. In the case of capacitive dielectric sensors (see, U.S. Pat. No. 4,924,702 and U.S. Pub. App. No. 2009/0148306 A1), two or more plates are electrically coupled to the medium under test, and typically the dielectric constant of a medium is inferred by changes to frequencies of oscillators that use the coupled capacitive plates as an oscillator element, or changes to rise time of pulses across the coupled capacitive plates. In the case of transmission line dielectric sensors (see, U.S. Pat. Nos. 3,995,212 and 5,610,611), electrical pulses are sent down a transmission line of fixed length which is electrically coupled to a medium under test, and the transit time of the reflected pulses are detected, and then correlated to the dielectric constant, as it is a function of the reflected pulse time.
Alternatively, the dielectric constant of a medium can be measured using transmission lines with known length and known end load and coupled to the medium under test (see, US Pub. App. Nos. 2009/0134889 and 2015/0323372). In these structures, the dielectric constant may be measured by electrically stimulating the line with a repetitive signal to determine the characteristic impedance of the transmission line segment at the transmission source, and from this characteristic impedance inferring the dielectric constant. These sensors often employ an AM peak detector circuit to measure the characteristic impedance of the transmission line segment. This structure of sensor may be utilized for many different purposes. For instance, the immediately above referenced applications disclose both a moisture content sensor and a liquid level sensor, respectively.
It is common for dielectric sensors to employ cables of wires for powering the sensors from a remote power source, and for reporting data back to a remote data logger or reader. It has been observed that, for transmission line based dielectric sensors in particular, measurements can be adversely affected by cable length, coils in the cable, and the proximity of the cable to other dielectric materials including humans touching the cable.
In view of the foregoing, there is a need to provide a dielectric sensor apparatus that is insensitive to cable length, cable coiling, and cable proximity to other dielectric materials.
The present invention is a transmission line dielectric sensor which utilizes a cable filter to reduce the sensor's sensitivity to extraneous dielectric feedback. The present invention represents a departure from the prior art in that the dielectric probe of the present invention allows for use while cancelling the effects of cable length, cable coiling, and cable proximity to other dielectric materials.